This invention relates to a semiconductor device in which a semiconductor element is bonded to a support structure and a process for bonding a semiconductor element to a support structure.
The die bonding for use in bonding a die (semiconductor element) to a semiconductor element support structure such as a package or a substrate typically includes a gold-silicon eutectic bonding and a solder bonding. In recently developed devices such as color image sensors including a color filter which are less heat resistant applying the low-viscosity bonding agent while spinning the package directly formed on a semiconductor element, such as infrared ray detectors in which the semiconductor element is used at a cryogenic temperature as low as nitrogen liquidifying temperature (77.degree. K.).
In the die bonding, the color image sensor, the gold-silicon eutectic bonding method, or a solder which needs an elevated temperature cannot be used since the color filter used is not heat-resistant. Therefore, a bonding agent containing silver powders or a bonding agent of the type cured by ultraviolet rays is used. In the infrared ray detector, formed by the gold-silicon eutectic bonding method or a solder material are too rigid to accommodate the difference between the thermal expansions of the semiconductor element and the container vessel or the package. These bonds cause destruction of the semiconductor element at the bonded portion. Therefore, a low-temperature bonding agent which is relatively flexible and strong at low temperatures is used in the infrared ray detector.
The low-temperature bonding agents generally used are an epoxy or a urethane bonding agent curable at room temperature. Such bonding agents include Hisol (trade name) and Crest (trade name). These low-temperature bonding agents when cured have a sufficient flexibility or softness as well as a sufficient adhesion strength at cryogenic temperatures. However, since these bonding agents before curing have a high viscosity, of from 2,500 cps to 100,000 cps, it is very difficult to apply such the bonding agent to the surface to be bonded in a uniform thickness.
FIG. 1 illustrates, in cross section, a conventional semiconductor device 1 of the type to which the present invention is applicable. The semiconductor device 1 comprises a semiconductor chip 2, a support structure which is illustrated as a package 3 for containing therein and supporting the semiconductor element 2, and a bonding layer 4 of a bonding agent disposed between the semiconductor element 2 and the package 2 for bonding the semiconductor element 2 on the package 3. The bonding layer 4 is applied on the package 3 with a manual applicator (not shown) such as a putty knife or a brush to form a bonding surface 5 bonded to the package 3 and a bonding surface 6 to which the semiconductor element 2 is to be bonded. The bonding agent of the bonding layer 4 has a relatively high viscosity, so that the bonding surface 6 of the layer 4, before the semiconductor element 2 is placed on it, is an irregular free surface. This irregular free surface has projections 7 and cavities 8. Then, the semiconductor element 2 is pressed onto the bonding surface 6 of the bonding layer 4, and the bonding agent of the bonding layer 4 is cured.
The semiconductor device 1 thus manufactured has a number of cavities 8 or voids at the interface between the semiconductor element 2 and the bonding layer 4 because the relatively hard, highly viscous bonding agent of the bonding layer 4 is manually applied. Therefore, the bonding surface 6 of the bonding layer 4 only partially contacts and adheres with the semiconductor element 2, providing only a limited contact area between the die bonding region of the semiconductor element 2 and the package 3.
The voids formed by the cavities 8 between the semiconductor element 2 and the bonding layer 4 lower the thermal conduction between the semiconductor element 2 and the package 3. Also, during the thermal cycling of the semiconductor device 1 while in use, the moisture trapped within the voids 8 is repeatedly evaporated and condensed and the air pressure within the voids 8 changes. These factors significantly degrade the reliability of the semiconductor device.